This invention relates in general to sealing systems for liquid storage tanks and, in particular, to a sealing system for preserving the integrity of liquid contianed in a liquid storage tank, and limiting the evaporation into the environment of the contents contained therewithin.
More specifically, but without restriction to the particular use which is shown and described for purposes of illustrating a preferred embodiment of the invention, this invention relates to a seal for sealing the annular opening or rim space between a floating roof or cover and the inner wall or shell of a liquid storage tank containing a liquid upon which the roof or cover floats.
Liquid storage tanks for containing large quantities of a stored liquid, such as petroleum products, are generally constructed either by welding or riveting steel sections together to form a liquid-tight cylinder having an open upper end which is closed by a floating roof or cover which floats upon the liquid contained within the storage tank and is appropriately sealed to prevent contamination of the liquid contained within the tank, and the escape of vapors from the tank into the atmosphere. The sealing systems for such tanks are exposed to the elements and must be designed such that the seals will resist wind loading forces applied to the tank which move the floating roof within the confines of the internal wall or shell of the storage tank. Such seals not only must exert a sufficient countering force against the inner wall of the tank to maintain seal integrity on both the upwind and downwind side, but must accommodate the eccentricity of the tank walls, which frequently results in the floating roof and the inner wall or shell of the tank being non-concentric.
One attempt to provide a suitable seal for the rim space between the floating roof or cover and the inner wall of the tank shell has been the use of a shoe-type sealing system which seals the rim space annulus between the floating pontoon and the inner walls of the tank. Shoe-type sealing systems utilize a hanger-pusher assembly to hold the floating pontoon away from the inner wall of the tank, while applying a centering pressure against the tank shell. To his end, a shoe plate forms a horizontal ring about the pontoon, between the pontoon and the inner wall of the tank shell. The shoe plate slides up and down the inner wall of the tank shell as the pontoon rises and falls with variations in the quantity of the liquid stored within the storage container. A curtain or fabric seal extends from the shoe to the floating roof or pontoon to close the annular rim space.
While such shoe-type seals are suitable for certain types of applications, such seals may not be flexible enough to compensate for tank shell irregularities and, therefore, are frequently incapable of maintaining the vapor seal integrity and the permissible maximum gap requirements set forth by state and federal codes and regulations. As a result of this inability to compensate for tank shell irregularities, such sealing systems may discharge excessive vapor into the atmosphere.
Resolving the problem of tank shell irregularities is even more difficult when providing a seal for a liquid storage tank which is fabricated by riveting sections of steel plate together. In such applications not only is tank eccentricity encountered, but the vertically and horizontally disposed rivets used in fabricating the tank produce irregularities in the inner wall of the tank shell which are encountered by the sealing system as the floating roof moves vertically within the tank sheel in response to variations in the quantity contained within the storage tank. One such attempt to construct a seal especially suitable for use with a storage tank of riveted construction is disclosed in the inventor's prior patent, U.S. Pat. No. 4,273,250 "SEALING SYSTEM FOR LIQUID STORAGE TANKS". In this patent, the inventor has disclosed a sealing system utilizing a seal having an "E"-shaped cross section with a plurality of outwardly extending flanges positioned adjacent to shorter flanges which function as a stiffening or brace to prevent the longer sealing flanges from bending down into contact with each other, or too far so that they might lose their sealing function. The present invention is an improvement of the inventor's prior sealing system.